Around Washington, Rep. William J. Jefferson nurtured a reputation as a serious, even wonkish, lawmaker, a grade-school dropouts' son who graduated from Harvard Law School and was elected Louisiana's first black congressman since Reconstruction.

Then came the allegations last August that Jefferson had orchestrated a corruption scheme. Federal investigators are targeting the Democratic congressman, 58, for allegedly demanding cash and other favors for himself and relatives, in exchange for using his congressional clout to arrange African business deals. A former aide recently pleaded guilty to bribing Jefferson and is cooperating with authorities, and sources familiar with the case say a plea agreement with the lawmaker is being explored.

Jefferson's world is toppling. Tall and lean, he at times has looked ashen as he walks the halls of the Capitol. Those who know him describe him as shellshocked by the turn of events. Depending on Jefferson's fate, his central New Orleans district -- badly damaged by Hurricane Katrina and in need of effective representation in Washington -- could face a rowdy special election. The political scene is so chaotic that Republicans believe they could win the gerrymandered Democratic seat.

"It's all clear as mud," said Edward F. Renwick, a Loyola University political scientist.

Jefferson's woes are unwelcome news for his party and have undercut the Democrats' election-year assertion that Republicans have created a "culture of corruption." If Jefferson is indicted and pleads guilty or is convicted, he will have to step down or face expulsion. But if he is indicted and decides to go to trial, he may remain in Congress and stand for reelection -- the course Rep. Tom DeLay (R-Tex.) has followed since being charged last year with violating Texas campaign law.

Federal corruption investigations have produced guilty pleas from former representative Randy "Duke" Cunningham (R-Calif.) and have forced Rep. Robert W. Ney (R-Ohio) to relinquish his committee chairmanship. Investigations also won guilty pleas and the cooperation of former Republican lobbyist Jack Abramoff, whose plea agreement cited only GOP aides and lawmakers.

The investigation of Jefferson and the recent guilty plea by a former aide give Republicans the chance to argue that corruption in Washington has a bipartisan tinge.

Republican groups frequently invoke the Jefferson case in defending their party from broad-brush charges of corruption. Even Public Citizen, a liberal consumer watchdog group, featured Jefferson on an "Ethics Hall of Shame" list recently.

Jefferson has said he did nothing improper. Spokeswoman Melanie Roussell said he is focused on hurricane recovery and has been traveling to his district for field hearings and other storm-related events. Jefferson has begun campaigning for election this fall to a ninth term and has scheduled a March 8 fundraiser. He attended Coretta Scott King's funeral in Atlanta last week.

In his only public statement on the case, Jefferson said he was "disappointed and in some ways perplexed" by former aide Brett Pfeffer's guilty plea on Jan. 11. Jefferson added that he has never "required, demanded or accepted . . . anything to perform a service for which I have been elected." Ron Machen, one of Jefferson's attorneys, declined to comment.

The investigation became public on Aug. 3 when FBI agents raided Jefferson's homes in New Orleans and Northeast Washington, where they found about $90,000 in cash in his freezer, law enforcement sources have said. They also raided five other locations, including the Kentucky and New Jersey offices of iGate Inc., a high-tech firm that has become central to the investigation, along with a house in Potomac owned by Atiku Abubakar, the vice president of Nigeria.

IGate has denied any wrongdoing, as has Abubakar. Pfeffer declined to comment.